Sweden is great for outdoor life - skiing, skating, hiking, canoeing, cycling and berry-picking depending of season. Stockholm and Gothenburg have great nightlife and shopping opportunities. Most cities have well-preserved pre-industrial architecture.
Swedish weather is best during the summer (late May to early September). If you like snow, go to Norrland or Dalarna in November to April.
Be aware that daylight varies greatly during the year. In Stockholm, the sun sets at 3 PM in December. North of the Arctic Circle one can experience the midnight sun and Arctic night. However, even at Stockholm's latitude, summer nights exist only in the form of prolonged twilight during June and July.
The major holidays are Easter, Midsummer (celebrated from the eve of the Friday between June 19 - 25), Christmas (Christmas Eve, Christmas Day and Boxing Day are all considered holidays), and the "industrial vacation" throughout July. Expect closed establishments, heavy traffic (for the holidays) and crowded tourist resorts (for July).
Note that most Swedish holidays are celebrated on the day before (Midsummer's Eve, Christmas Eve etc), while Swedish people do hardly anything on the holiday proper.